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How are ore body sizes determined? What are the most reliable techniques of determining the size of ore deposits as well as their content?

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The initial stage of mineral exploration involves finding a site to investigate. In some situations this may result from the discovery of a surface outcrop of mineralization, which is the exposure of a mineral deposit.

Surface outcrops are becoming more difficult to find because some have already been mined or they are buried under sediments or lava flows. To get an indication where buried mineral deposits occur, geophysical exploration techniques are required. Generally this will require the use of either gravity measurements and/or magnetic surveys, usually aeromagnatic surveys. In some situations geochemical exploration methods may be used as well. This requires wandering over land taking samples of the surface rocks and assaying them for minerals and metals. The assaying method used will be determined by the metal being explored for. Fire assays, when they are used, are used for gold, not base metals.

When a prospective area has been found, exploration drilling is undertaken. Initially reverse circulation drilling may be done because it is cheaper and quicker. The cuttings from the drill holes are analyzed by a geologist to ascertain the rock types and samples of prospective rock types are assayed for their metal content.

If the mineral deposit is large enough and rich enough, diamond drilling will be undertaken. Depending on the condition of the ground being drill, this generally provides a solid core of rock that can be inspected for rock type. Usually the core is split in half lengthwise and half the core for prospective rock types are assayed for metal content.

To get an accurate understanding of the size the mineral deposit and the amount of metal present. A grid of holes, drilled on a 20 m by 20 m or a 10 m by 10 m pattern is drilled over the deposit. The length of the drill holes will largely be determined by the depth of the mineral deposit. The results of this, outlines the size of the deposit.

The assays for metal content and subsequent modelling by geologists determines the amount of metal in the deposit (its richness). This may involve the use of geostatistics.

Classifying the deposit as either a mineral deposit or orebody is done by mining engineers who evaluate the models created by the geologists by creating possible mine designs and evaluating the economics of the potential mine. If the deposit is economic to mine is called an orebody, if not it remains as a mineral deposit.

As part of the mine design and evaluation process, the mining engineers must engage with metallurgists regarding how any ore from the deposit will be processed and the costs involved. In developed countries they must also consider the rehabilitation of any potential mine, once it has been mine out, during the evaluation of the mineral deposit.

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